A polymer is a large molecule (macromolecule) composed of repeating structural units. These subunits are typically connected by covalent chemical bonds. Although the term polymer is sometimes taken to refer to plastics, it actually encompasses a large class comprising both natural and synthetic materials with a wide variety of properties.
Most commonly, a polymer is a molecule comprising a continuously linked backbone of a polymer consisting mainly of carbon atoms. An example is polyethylene whose repeating unit is based on ethylene monomers and exists in various forms of various molecular weights. Oxygen is also commonly present in various polymer backbones, such as those of polyethylene glycol, polysaccharides (in glycosidic bonds), and DNA (in phosphodiester bonds).
Polymers can have different lengths and molecular weight assume different structures based on the arrangement and microscale ordering of the polymer chains in space. Many factors affect structure formation including molecular weight and chemical nature of the repeating units and therefore achieving control of polymer morphology has been challenging, in particular with reference to synthesis of polymers having high molecular weight.